This invention relates to a novel contact construction for a circuit breaker wherein a pair of main contacts of relatively high conductivity are arranged in parallel with arcing contacts which have a steel body of relatively low conductivity.
Circuit breaker contact configurations are well known to those skilled in the art. For example, an air-magnetic breaker which may have a voltage rating of above 1,000 to about 20,000 volts and a continuous current rating in excess of 1,000 amperes will conventionally have a movable contact which carries a main contact which makes low resistance connection to a cooperating relatively stationary main contact and will also have a movable arcing contact which makes connection with a cooperating arcing contact on the relatively stationary body. The arcing and main contact sections are electrically connected in parallel with one another.
The contacts are then arranged in such a manner that, when opening the contacts, the main contacts will open first so that the current through the main contacts will commutate into the parallel-connected arcing contacts, and the arcing contacts are thereafter opened, with all arcing duty being handled by the arcing contacts. By using this configuration, the main contacts are kept relatively smooth and unpitted and unaffected by arcing problems so that a good low resistance connection can be maintained between the main contacts even after numerous operations. Moreover, this permits the main contacts to be made of materials best adapted for normal low resistance connection to one another without concern for ability to withstand high arcing temperatures or high mechanical strength.
The arcing contacts on the other hand are selected to be of material which is highly resistant to deterioration due to arcing and arcing products with relatively little concern for contact resistance, since they can have a relatively high contact resistance while still allowing the commutation of current into the arcing contacts after the main contacts have opened.
The arcing contacts of air-magnetic circuit breakers have traditionally been made of a non-ferrous high conductivity high mechanical strength material. For example, to provide the thermal capacity and mechanical strength which was thought necessary for the arcing contacts, the material usually used has been a copper alloy such as beryllium-copper or at least silver-bearing copper which has high resistance silver molybdenum arcing contact tips brazed to the beryllium-copper body. As previously noted, since the arcing contacts will be connected in parallel with high conductivity main contacts, the continuous current capacity of the arcing contacts is relatively unimportant and the relatively high resistance materials are used because they have the desired thermal and mechanical characteristics.
The materials which have traditionally been used for arcing contact bodies, such as a beryllium-copper alloy, is relatively expensive and is relatively difficult to machine and otherwise process. Circuit breaker manufacturers, however, have traditionally continued to use relatively expensive beryllium-copper alloys for the arcing contact bodies since, in part, it was believed necessary to use a non-ferrous material so as not to interfere with the movement of the arc between the arcing contacts.